3) AND who's parent has '0' as one it's fno's in the child_tbl2 Therefore, id 4, 6 & 7 are eliminated from the existing results, this is because 0 was not found in one of their fno's, while 0 was found as one of xid 1's fno

If it is possible, I do not know how to tell the dbms (MySQL) to check if Zero is one of the fno's. If I say "AND child_tbl2.fno = '0'", I will be saying that the result's fno should be 0. I do not want that, I need zero to be one of the fno's in order for the query to SUM all the qnty in that particular xid